An LED is a p-n junction diode that can emit ultraviolet, visible and infrared light. A visible LED is usually used as the light source of the operation panel for electric appliances such as, for example, the light source of a camera with an auto-focus function and the light source of a bar code reader.
A visible LED is an LED that can emit visible light with a wavelength of 400 nm to 700 nm. A visible LED can be manufactured by utilizing III-V semiconductor materials having energy gaps within the range of 1.36 eV to 3.26 eV, such as GaP, Ga1−xAlxAs, GaN, and GaAs1−yPy.
The brightness of a visible LED is the most important quality for use thereof. Some manufacturing steps can be performed to enhance the brightness of a visible LED, such that a transparent electrode layer is added before forming a metal electrode during the manufacturing process of a visible LED. Suitable materials for the transparent electrode layer are used, such as InO, SiO, ZnO, or ITO (Indium Tin Oxide). The transparent electrode layer can be used not only to form ohmic contact between the LED devices, but also to diffuse the electricity flowing therethrough to enhance the brightness of the visible LED. However, the conventional procedure for forming the transparent electrode layer, by depositing suitable materials directly on the LED element, forms a proper ohmic contact between the LED devices only with difficulty.
To resolve the problem, some III-V semiconductor materials, such as GaAs, GaP, or GaAsP have been used to form a p-type ohmic contact film with heavy doping between the LED element and the transparent electrode layer to reduce the resistance of the transparent electrode layer. However, when GaAs is doped into the AlGaInP-based LED element to form the ohmic contact film, most of the visible light emitted from the LED element can be absorbed by the GaAs because the energy gap of the GaAs, about 1.35 eV, is less than 1.63 eV to 3.26 eV, which is the energy gap of visible light. Although using other materials avoids the energy gap problem, electric resistance still increases.
On the other hand, a hybrid superlattice structure of the contact layer of the LED element has been used as an alternative way to reduce the electric resistance of the transparent electrode layer. However, the structure still absorbs visible light.
A problem in this technique is that it can be difficult to fabricate a transparent electrode with low resistance that also does not absorb visible light.